Arabic numbers were a great
labor saver when they began to enter into medieval Europe ca.
1200!

Previously, medieval European
mathematics were based on Roman numerals, which are straightforward
enough to add (at least for small quantities), but extremely difficult
to multiply or divide. Such calculations were traditionally performed
using an abacus and counters. Arithmetic was a complex subject,
taught to mature young men, not schoolboys.

With
the introduction of Arabic numerals, one begins to see Europeans
undertaking more and more complex calculations. One example is
long multiplication, for which the jalousia (or gelosia)
method was used. The method is so called after the iron grill,
or jalousia, Italian men would place over their windows
to keep strangers from staring at their wives. It is also called
lattice multiplication.

Above is an example, from around
1300, of how to multiply4,569,202 (across the top of the grid)
by 502, 403 (down the right side). To begin, each digit of the
multiplicand is multiplied separately with each digit of the multiplier,
and the product is recorded in the corresponding split square.
To complete the calculation, you simply add the diagonal columns
from top right to bottom left to yield 2,295,570,802,406.

Try it yourself! You might prefer
this method to the one you were taught.

Lattice multiplication first
was introduced to Europe by Fibonacci
(Leondardo of Pisa), whose 1202 treatise Liber Abacii (Book
of the Abacus) was the most sophisticated work on arithmetic
and number theory written in medieval Europe.

When my father, who had been appointed by his country as public
notary in the customs at Bugia acting for the Pisan merchants
going there, was in charge, he summoned me to him while I was
still a child, and having an eye to usefulness and future convenience,
desired me to stay there and receive instruction in the school
of accounting. There, when I had been introduced to the art
of the Indians' nine symbols [that is, Arabic numerals] through
remarkable teaching, knowledge of the art very soon pleased me
above all else and I came to understand it, for whatever was studied
by the art in Egypt, Syria, Greece, Sicily and Provence, in all
its various forms. --Fibonacci, Liber Abaci (1202)